Galaxies in the observable universe
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Galaxies in the Observable Universe
Galaxy Clustering and Cosmological Probes
Galaxy clustering provides critical insights into the large-scale structure of the universe. The observed redshift and position of galaxies are influenced by matter fluctuations and gravity waves between the source galaxies and the observer. This results in additional contributions to the observed galaxy fluctuation field, including tensor and scalar contributions. A general relativistic description of galaxy clustering is essential for accurate theoretical predictions and can be used to compute the angular auto-correlation of large-scale structures and their cross-correlation with Cosmic Microwave Background (CMB) temperature anisotropies. This approach also opens the possibility of detecting primordial gravity waves through galaxy clustering.
Observing High-Redshift Galaxies with JWST
The James Webb Space Telescope (JWST) is expected to revolutionize our understanding of galaxies at redshifts greater than 10, which are currently inaccessible. Using empirical models like the UniverseMachine, researchers can generate mock galaxy catalogues and lightcones over a redshift range of 0-15. These models predict that the number density of observable galaxies will increase significantly at redshifts greater than 12, providing valuable constraints on galaxy formation models. The faint-end slopes of the stellar mass and luminosity functions steepen with increasing redshift, indicating that observable galaxies are hosted by haloes in the exponentially falling regime of the halo mass function at high redshifts.
Large-Scale Distribution and Clustering of Galaxies
Contrary to earlier beliefs that galaxies are isolated and uniformly spread, recent observations suggest that clusters of galaxies are the norm. Space can be subdivided into "cluster cells," each occupied by one or more clusters of galaxies, filling the universe much like bubbles in suds. This clustering is a fundamental characteristic of the large-scale structure of the universe. The distribution of galaxies is a key source of information about the distribution of matter on very large scales. If galaxies are a statistically fair sample of the overall mass distribution, the universe must be open. However, if galaxies are overrepresented in high-density regions, the observational data may be compatible with a closed or flat universe. The hierarchical clustering model in a cold dark matter (CDM) universe suggests that the universe's density must approach the closure value, with a significant dependence of galaxy clustering strength on the depth of the potential well of the galaxies considered.
Morphological Classification and Evolution of Galaxies
Galaxies in the observable universe are categorized into three main morphological types: spirals, ellipticals, and irregulars. This classification helps in understanding the formation and evolution of galaxies over cosmic time. Observations reveal that galaxy structures have evolved from small, compact, and peculiar systems in the distant universe to the well-known Hubble sequence dominated by spirals and ellipticals. Structural analyses, including methods like Sersic fitting and nonparametric structural indices, provide insights into the scale, star-formation rate, and merger activity of galaxies. These methods also help identify galaxies in mergers and measure their merger history up to redshift 3 .
The First Stars and Galaxies
Large telescopes have enabled astronomers to observe galaxies that formed as early as 850 million years after the Big Bang. The first observable star likely formed 30 million years after the Big Bang, and the first galaxy as massive as the Milky Way likely formed when the universe was only 400 million years old. These observations require significant modifications in current methods of simulating galaxy formation at high redshifts.
Conclusion
The study of galaxies in the observable universe provides profound insights into the structure, formation, and evolution of the cosmos. From the clustering of galaxies and the role of dark matter to the morphological classification and the observation of high-redshift galaxies, each aspect contributes to a comprehensive understanding of the universe's history and its underlying physical laws. The advancements in observational technology, such as the JWST, promise to further unravel the mysteries of galaxy formation and evolution, offering a clearer picture of our universe's past and future.
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